This invention is related to non-ferrous hydrometallurgy and can be used in those plants wherein nickel and cobalt ores, concentrates and intermediate products are treated by the ammonia-ammonium carbonate technology, after the previous nickeliferous ore direct reduction (Nicaro, Punta Gorda, Queensland Nickel, Marinduque plants, among others), or in plants newly constructed. A more specific application could also be to the processing of intermediate products or nickel and cobalt concentrates amenable to leaching by an ammonia-ammonium carbonate solution.
The Northamerican Patent No. 1847145, dated Mar. 18, 1924, Author: M. H. Caron, establishes the selective reduction with respect to iron, leaching of the reduced ore with ammonia-ammonium carbonate solution, countercurrent washing of the leached pulp in thickeners, the optional removal of cobalt in the form of sulphide, distillation of the pregnant liquor or solvent extraction. The final product may be nickel metal (briquette, powder, oxide) Class I or II, cobalt metal (powder, salts, sulphides), among others.
The known procedure comprises ore grinding to 80-85% xe2x88x9274 xcexcm, either mixed with 2-4% petroleum, or reduction by a reducing gas (CO+H2) in a multi-hearth furnace. Both the atmosphere and temperature inside the furnace are controlled to attain a high reduction of nickel and cobalt and a very low one of iron. Generally, nickel is metallized up to 95-98%, and cobalt to 75-80%, which is further extracted by ammonia leaching. High metallic iron contents prevents the process to develop satisfactorily, due to the heat released during its oxidation and, consequently, the pulp runs hot in 10-12xc2x0 C., thus requiring to be cooled, as well as the liquor, in such a magnitude.
The formation of Fe(OH)3 throughout this procedure, arises nickel (1-5%) and cobalt (20-40%) losses because of their readsorption on the precipitate. At Queensland Nickel this is a single-stage process carried out in a bank of large turboaerators, at a high ammonia concentration (95 g/L against 60-65 g/L NH3 at Nicaro), whereby nickel and cobalt losses were reduced in 0.5% and 15-20, respectively.
In the traditional process leaching is performed for 1.5-3 hours, and a considerable number of turboaerators is required (i.e. 66 turboaerators at Punta Gorda in the Cuban plants, and 12 huge turboaerators for Queensland), thus implying high energy, ammonia, and air consumption and maintenance costs for both cases.
Traditional process at low ammonia concentrations generally includes three stages: each of these stages includes leaching in turboaerators with liquid/solid separation in a thickener.
Liquid/solid separation generally takes 1 or 2 days, and arises metal losses due to their coprecipitation and readsorption on iron hydroxide and manganese oxides.
Throughout said process cobalt is partly precipitated between the first and second stage, either using ammonium hydrosulphide or sodium sulphide (or H2S). The nickel-rich liquor is subjected to distillation to produce basic nickel carbonate, and nickel of different qualities is then obtained as final product.
At Queensland, the nickel and cobalt-rich product liquor with high ammonia contents (90-95 g/L) is removed, nickel by solvent extraction and the cobalt-rich liquor (raffinate) is precipitated by ammonium hydrosulphide. The final product is a combined sulphide rich in cobalt (Co:Nixe2x89xa740:1). The final product is nickel Class I and cobalt sulphide. Solvent LIX with selectivity for nickel is used in this process.
At Tocantis, nickel and cobalt are precipitated by distillation, and the combined carbonate (Ni+Co) is dissolved in sulphuric acid, pure metals being further removed by solvent extraction.
On the other hand, most of the laterite nickel ores exhibits a magnesium content relatively high, and are not treated by pressure acid leaching at high temperature, mainly because it is unprofitable, requires costly investments for magnesium oxide removal and recycling, and also due to the build up of large crusts on the plant circuit.
The present invention is aimed to develop a hydrometallurgical extraction procedure for the intensive leaching of nickel and cobalt-bearing ores in a lesser time than the traditional process, capable of providing an operation at a low ammonia concentration, and leaching all the extractable nickel, as well as all the cobalt leached and coprecipitated together with iron hydroxide.
Another objective envisaged to the invention is the reduction of investment cost and production costs as to supplies, ammonia, energy and maintenance, and also to facilitate the complete automation of the leaching process.
An additional goal of the invention includes the development of a hydrometallurgical extraction procedure for the intensive leaching of concentrates or intermediate products containing nickel and cobalt in a soluble form in ammonia solutions.
A hydrometallurgical procedure for nickel and cobalt recovery by the intensive leaching of ores, concentrates or intermediate products, which involves the basic stages and other additional ores, constitutes a novelty of the invention.
The basic stages are:
a. The materialxe2x80x94a nickeliferous ore bearing 0.5-3% nickel, 0.005-1.5% cobalt and 10-55% ironxe2x80x94is submitted to a selective reduction process at a temperature range between 540 and 580xc2x0 C., using a reducing agent.
b. The material which can be an intermediate product or a nickel and cobalt concentrate, wherein the metals contained (Ni and Co) are soluble in ammonia solutions, and a previous reduction is not necessary.
c. Reduced ore is cooled in the presence of reducing or inert gases.
d. Reduced ore or the nickel and cobalt intermediate product or concentrate is brought into contact with an ammonia-ammonium carbonate solution, in such a way that pulp temperature does not exceed 60xc2x0 C.
e. Pulp is leached in a tubular reactor to which air or oxygen is injected over a leaching time between 5 seconds and 15 minutes, with a high oxygen adsorption.
f. Pulp is cooled in the tubular reactor, where temperature just increases from 1 to 6xc2x0 C.
The procedure herein proposed gives solution to two fundamental problems: firstly, it allows a more effective reduction of the leachable nickel and cobalt up to 80-90% (Ni) and 70-80% (Co), and, second, iron metallization stops to be a limiting factor.
The mineral ground xe2x88x9274 xcexcm in a 60-90%, is reduced by a reducing gas with 2-50% (CO+H2), or by a mixture of a reducing gas and additive petroleum (0.5-4.5% by weight), or petroleum alone as reducing agent, which requires about 5-90 minutes. The mineral is cooled at a temperature of 130-280xc2x0 C.
When leaching is performed at low ammonia concentrations (60-65 g/L) and CO2 (30-35 g/L) likewise to the traditional process, nickel and cobalt recovery is increased in 2-6% and 30-40%, respectively. Heat release is offset by pulp self-cooling, resulting in just an increment of 3-4xc2x0 C.
This procedure practically eliminates 75-90% of the turboaerators, performance time is below 15 minutes, and operates at low ammonia concentrations, although it is also possible to operate at high concentrations. Consequently, the cost relative to investment and air, ammonia, energy consumptions and maintenance are lowered, and besides offers the possibility of a complete automation.
An advantage of the invention is that iron oxidation to oxide takes place quickly, thus iron hydroxide formation is reduced and, hence, metal losses.
The procedure object of the invention behaves more efficiently in comparison to the traditional one even though at a NH3 concentration below 60 g/L, which is not observed in the latter. On the other hand, our procedure involves the flash separation of the solid from the pulp, using hydrocyclones, whether combined or not with a thickener, or a high-productivity thickener, thus reducing Ni and Co losses.
The procedure as shown in FIG. 1xe2x80x94the reduced ore from the ore cooler is mixed with recycling liquor (1) containing ammonia (30/120 g/L-30/80 g/L CO2) and CO2 or ammonium sulphate liquor and ammonia at equal ammonia concentrations, and a Ni concentration (up to 13 g/L) and Co (up to 0.5-0.69 g/L) in a duct going to the contact tank (2); pulp temperature is below 60xc2x0 C. and density is in the range of 1050 and 1250 g/L. The pulp passes through the tubular reactor by means of a pump (3) at a spatial rate between 0.5 and 29 m/s; an air flow or other oxidant is injected to the reactor at a spatial rate between 0.25-15 m/s.
The pulp is discharged into a thickener (6) for liquid/solid separation, a part of the pregnant liquor rich in Ni and Co is passed through the liquor coolers (7) to the duct or contact tank (1), and the other one is sent to Ni and Co separation (8). The thickener underflow (9) can be submitted to a second leaching stage, if necessary, or to a countercurrent washing system (CCD) for the reextraction of 0.5-10% nickel and 2-25% cobalt contained in the solid.
The outlined method comprises one or various additional stages as to leaching, liquid/solid separation, solvent or resin extraction which allow to increase the metallurgical efficiency of the process (nickel and cobalt recovery), and to reduce investments for using less equipment.